1. Field of the Invention
The present invention relates to an improved electromagnetic levitation type continuous metal casting apparatus.
2. Description of the Related Art
Conventionally, wires and rods composed of Al or Cu are produced by a continuous metal casting method as disclosed in U.S. Pat. No. 4,414,285. In this method, molten metal in a column shape is upwardly supplied to an upper casting or forming area. Thereafter, the molten metal column is exposed to an alternating electromagnetic levitation and containment field while being moved upwardly in the casting or forming area by withdrawal rolls. Simultaneously, the molten metal column is successively cooled and solidified, and the solidified metal product thereafter is removed from the top of the casting or forming area This electromagnetic levitation type continuous metal casting method has been practically used as an industrially effective means. According to the aforementioned electromagnetic levitation type continuous metal casting method, molten metal column to be cast or formed can be readily removed free from frictional forces and bonding forces against the sides of a casting vessel (mold) because the aforementioned alternating electromagnetic levitation and containment field produces a gravity free state referred to as "pressureless contact". In addition, in such a method, while the molten metal column passes through the alternating electromagnetic field, the inside of the molten metal column is stirred and thereby high homogeneity can be accomplished.
As an apparatus using the aforementioned continuous metal casting method as shown by a sectional view of FIG. 1 has been known. This apparatus comprises a molten metal storing furnace 2 for storing and holding a molten metal 1, a tube shaped casting vessel 3 vertically disposed for receiving the molten metal in the form of a column so as to solidify the molten metal 1, a heat exchange means 4 unified with the casting vessel 3 for cooling and solidifying the molten metal column received into the casting vessel 3, an alternating electromagnetic field generation means 5 composed of a plurality of layers of coils and disposed on almost all the periphery of the casting vessel 3 for generating the alternating electromagnetic levitation and containment field that acts on the upwardly moving the molten metal column, a means 6 such as withdrawal rolls for removing the solidified metal product which has been cooled and solidified from the top of the casting vessel 3, a molten metal supplying path 7 (named a rounder tube) for upwardly supplying the molten metal to be cast from the molten metal storing furnace 2 into the casting vessel 3, the molten metal supplying path 7 being a graphite tube with a high frequency heating means 8 disposed on the periphery thereof, and a liquid level adjusting unit 9 for adjusting the liquid level of the molten metal 1.
However, in the aforementioned electromagnetic levitation type continuous metal casting method, there are following problems to be solved.
As one of the problems, since the molten metal supplying path 7 for upwardly supplying the molten metal 1 to be cast from the molten metal storing furnace 2 into the casting vessel 3 should successively supply the molten metal 1 while keeping it in a particular molten state, the graphite pipe with high conductivity is used and the high frequency heating means 8 is disposed on the periphery thereof. However, the molten metal supply path 7 extends through the casting vessel 3 which is vertically disposed. At a bend section (elbow section) 7a, it is difficult to accomplish enough turns of a coil structuring the high, frequency heating means 8. Thus, the molten metal 1 cannot be always kept in the particular molten state. In other words, when the molten metal is supplied at a relatively low speed so as to perform a low speed casting operation, since the molten metal being supplied is solidified or cooled at the bend section 7a, the required amount of the molten metal 1 cannot be continuously supplied. Thus, in the molten metal supplying path 7, an improvement of the apparatus for continuously supplying the molten metal 1 has been required.
As the second problem, in the electromagnetic levitation type continuous metal casting apparatus in the aforementioned structure, as shown in FIG. 2 which is an enlarged sectional view of the principal portions of the casting vessel of FIG. 1, the casting vessel 3, the heat exchange means 4, and the alternating electromagnetic field generation means 5 are unified. In other words, on the outer periphery of the tube shaped casting vessel 3 with a fire proof layer 3a such as a graphite liner or the like disposed on the inner wall thereof, a flow path of a coolant (heat exchange means) is unified. In addition, in the full length of the outer periphery of the flow path of the coolant (heat exchange means) 4, a plurality of electromagnetic levitation coils (alternating electromagnetic field generation means) 5 are disposed. In such a structure, the first cooling point becomes a bottom plate 4a of the heat exchange means 4. When the alternating electromagnetic field generation means 5 is composed of six layers of coils 5a, required strength of the levitation electromagnetic field is obtained in the area of the second layer from both the ends thereof.
However, in the aforementioned electromagnetic levitation type continuous metal casting apparatus, there is the following problem. The molten metal column supplied upwardly from the molten metal storing furnace 2 for storing the molten metal 1 into the lower side of the casting vessel 3 through the molten metal supplying path 7 is cooled and solidified by the heat exchange means 4. At that time, the molten metal column is electromagnetically and upwardly levitated by the alternating electromagnetic field generation means 5 and then desired cast products, such as, wires are continuously produced. Thus, break aparts of the wire often take place. Such break aparts result from the fact that part of molten metal column supplied upwardly to the casting vessel 3 is solidified in the area or the lower area of a coil 5a1 which is the first layer from the bottom of the alternating electromagnetic field generation means 5, namely the area where levitating force and inwardly directed containment force cannot be satisfactorily obtained. Thus, the molten metal column is in contact with the wall of the casting vessel 3, thereby disturbing smooth upward movement of the molten metal column. To solve such a problem, in the wall area of the casting vessel 3 according to the coils 5a1 and the coil 5a2 which are respectively the first layer and the second layer from the bottom, a ceramic tube 3b is disposed, an air gap being disposed on the wall of the casting vessel 3 so as to decrease.. the thermal conductivity. However, in the aforementioned structure, the problem has not been solved.
The third problem is with respect to the molten metal supply path. As shown in FIG. 3, an apparatus with a displacer 9 has been used, the displacer 9 pressing the molten metal 1 in the molten metal storing furnace 2 so a to supply the molten metal 1 in the molten metal storing furnace 2 to the casting vessel 3 through the molten metal supply path 7. The molten metal supply path 7 is connected to a side wall in the vicinity of the bottom of the molten metal storing furnace 2. The molten metal supplying path 7 is composed of a horizontal section 7a, a vertical section 7b, and connection bend section 7c for connecting them. In this case, the molten metal supplying path 7 for upwardly supplying the molten metal 1 to be cast from the molten metal storing furnace 2 into the casting vessel 3 is generally composed of a graphite tube with high thermal conductivity and a heating means using high frequency heating method or the like, the heating means being disposed on the outer periphery of the graphite tube. The graphite tube is structured so that the molten metal supplying path 7 is easily oxidized and worn out by oxygen in the air or the molten metal 1. Namely, the durability of the graphite tube is low. Thus, since there are many joints between the horizontal section 7a and the molten metal storing furnace 2, between the horizontal section 7a and the vertical section 7b and between the vertical section 7b and the connection bend section 7c, the repair and replacement works become complicated. In addition, the possibility of leakage of the molten metal 1 increases. The possibility of the leakage of the molten metal at such joints is increased further by the hydrostatic pressure produced by the molten metal 1 during the required casting operation. In addition, in repairing and replacing the cooling means 4, the molten metal 1 in the molten metal storing furnace 2 should be removed or collected. This wastes the raw materials and increases the cost of the products. Therefore, an object of the present invention is to provide an electromagnetic levitation type continuous metal casting apparatus for decreasing or preventing the leakage of the molten metal 1 from the molten metal supplying path 7, the electromagnetic levitation type metal casting apparatus being free of the requirement for both the complicated repair and replacement works of the molten metal supplying path 7 and the loss of the molten metal 1 in the supply path 7 the molten metal storing furnace 2.